1. Field of the Invention
Example embodiments of the present invention are related generally to a system, modem, transmitter and method for improving transmission performance, and more particularly to a system, modem, transmitter and method for improving transmission performance by applying a time domain equalizing (TEQ) process.
2. Description of Related Art
In a coded orthogonal frequency division multiplexing (COFDM) system, portions of data may be encoded by frequency division multiplexing (FDM) based on a modulation/demodulation method. A plurality (e.g., hundreds) of carrier waves may be allocated to transmit the encoded portions of data. Instead of transmitting the encoded portions of data sequentially (e.g., not at the same time), the encoded portions of data may be simultaneously transmitted in the COFDM system. The simultaneous transmission may enable a wider symbol interval and/or may reduce an impulse noise effect (e.g., performance degradation due to impulse noise). The COFDM system may further decrease an inter-symbol interference (ISI).
An electromagnetic wave may be received through a plurality of propagation paths (e.g., due to reflections off of buildings in a city). An image signal (e.g., a video signal) carried on the electromagnetic wave may have a ghost effect (e.g., due to the differences in the received signal based on the different propagation paths) and an audio signal may include a higher level of noise (e.g., impulse noise).
The COFDM system may be implemented within a single frequency network (SFN). Transmission rates for carrier waves may be individually adjusted. The transmission rate adjustment may increase an overall transmission rate (e.g., as compared to a single carrier wave modulation scheme). The COFDM system may be used with digital signal processing (DSP) and very large scale integration (VLSI) technologies. For example, the COFDM system may be adapted to digital audio broadcast (DAB), terrestrial digital broadcast and/or a high definition television (HDTV) broadcast.
In conventional COFDM systems, data may be transmitted in units referred to as symbols. An impulse response may occur in the data transmission through an air (i.e., wireless) and/or a cable (i.e., wired) channel. Adjacent symbols may be affected by the impulse response. The impulse response may cause ISI. The ISI may delay data transmission. A guard interval may be inserted between the adjacent symbols so as to maintain a given interval between the symbols (e.g., which may reduce ISI).
FIG. 1A illustrates an example of a conventional transmission scheme 100.
FIG. 1B illustrates an example of a conventional transmission scheme 150 including a guard interval 160.
Referring to FIG. 1B, the guard interval 160 may include a zero value guard interval or a guard interval having cyclic prefixes (e.g., formed by replicating portions of the symbols).
If an interval between the cyclic prefixes is too short, ISI may occur (e.g., due to a longer channel impulse response as compared to the cyclic prefixes). If the interval between the cyclic prefixes is increased (e.g., to reduce the ISI), the data transmission rate may be reduced.
A time domain equalizing (TEQ) unit may be used to reduce the ISI. The TEQ unit may not require an increase to the interval between the cyclic prefixes.
A reception time domain equalizing (RX_TEQ) unit may be used to reduce reception channel impulse response in a discrete multi-tone (DMT)-based ADSL modem. According to the Asymmetric Digital Subscriber Line (ADSL) standard, the OFDM symbols may be constructed so as not to interfere with each other by using the cyclic prefixes. When the ADSL modem is used to transmit data through a channel having a shorter channel impulse response than the cyclic prefixes, a receiver may thereby be stabilized (e.g., since ISI between the OFDM symbols may be reduced).
The OFDM symbol between a transmission Inverse Fast Fourier Transform (IFFT) output port and/or a reception Fast Fourier Transform (FFT) input port may be affected by the channel impulse response as well as an impulse response due to digital and/or analog filters built in the transmitter and receiver. The channel impulse response of the OFDM symbol may thereby increase. Since the channel impulse response may be longer than the cyclic prefixes, ISI may occur between the OFDM symbols. Thus, the reception performance may deteriorate. If the TEQ process is performed so as to reduce the channel impulse response of the OFDM symbols, the ISI between the OFDM symbols may be reduced, which may thereby increase the reception performance.
The TEQ process may be applied to a cable-based streaming communication modem (e.g., an ADSL modem) with a continuous transmission of data. However, the TEQ process may not be capable of employment in a wireless packet communication modem (e.g., 802.11a modem, 802.11b modem, 802.11g modem, 802.16e modem, etc.).
A conventional operation of the TEQ process in a standard ADSL modem will now be described.
An impulse response of a reception channel may be estimated using a received signal. The received signal may be known at both of a receiver and a transmitter. In the ADSL standard, a pseudo-random sequence referred to as a reverb may be used. Namely, a known signal X1 may be transmitted by using the pseudo-random sequence (e.g., from the transmitter to the receiver).
A Finite Impulse Response (FIR) filter coefficient may be calculated in order to reduce the reception channel impulse response with the TEQ algorithm. Namely, assuming that a signal received by the receiver is Y1 with respect to the known signal X1, Y1=H*X1, where H=Y1/X1 is a constant corresponding to the channel impulse response, the TEQ coefficient may be given as 1/H=X1/Y1 (i.e., the reciprocal of the constant H).
The received signal may pass through the FIR filter to which the TEQ coefficient may be set. The channel impulse response of the received data may be reduced by using the TEQ process in the receiver. The ISI between the OFDM symbols may thereby be reduced and the reception performance may be increased.
FIG. 2 illustrates a transmission scheme of an ADSL modem receiver not including a TEQ process.
Referring to FIG. 2, a transmission DMT frame 200 transmitted by a transmitter may include the (n+1)-th cyclic prefix CPn+1 interposed between the n-th and (n+1)-th DMT symbols DMT SYMBOLn and DMT SYMBOLn+1. The reception channel impulse response 202 may be longer than the (n+1)-th cyclic prefix CPn+1. As shown, if the receiver does not include a TEQ process, the ISI between the n-th and (n+1)-th DMT symbols DMT SYMBOLn and DMT SYMBOLn+1 may occur in a reception DMT frame 204.
FIG. 3 illustrates a transmission scheme of an ADSL modem receiver including the TEQ process.
As shown in FIG. 3, when a transmission DMT frame 300 transmitted by a transmitter is passed through a reception channel, the channel impulse response 302 may increase. However, as described above, if the receiver includes a RX_TEQ process, a response signal 304 may include a reduced channel impulse response. Referring to the reception DMT frame 306, the length of the channel impulse response of the n-th DMT symbol DMT SYMBOLn may not be longer than the (n+1)-th cyclic prefix CPn+1. Thus, the ISI between DMT symbols may be reduced.
As described above, in a cable-based streaming communication modem (e.g., an ADSL modem), the TEQ coefficient may be calculated and repeatedly used until the communication is completed. However, in a wireless packet communication modem, data may be transmitted in units referred to as packets. The TEQ coefficient may require a new calculation for each transmitted packet. Conventional wireless modems may not support the TEQ process.
With respect to the ADSL standard, a given time (e.g., approximately 400 ms) may be allocated for the execution of the TEQ process. In wireless communication protocols (e.g., 802.11a), since the ISI between the OFDM symbols may be reduced by interposing the guard intervals between the OFDM symbols, the TEQ process may not be used.
In the 802.11a standard, the delay spread in an indoor environment may be smaller than the guard interval. In the 802.16 standard, the guard interval may be adjusted so as to cope with a longer delay spread (e.g., in an outdoor environment). However, in either indoor or outdoor environments, a longer delay spread may occur, which may degrade the reception performance. In addition, if the guard interval is adjusted to be longer in order to reduce the ISI between OFDM symbols, the transmission rate may be reduced by the increased guard interval.
Since the TEQ process may be applied to a reception data path of a modem, the TEQ process may be applied irrespective of the modem communication standard. For example, where the conventional TEQ process is applied to the 802.11a standard, reception channel estimation, a TEQ coefficient calculation and/or TEQ filter latency may require completion within a given interval (e.g., 8 μm). Further, the conventional TEQ coefficient calculation algorithm may require a higher amount of processing, which may require expensive hardware to implement.